Offshore marine structures such as production and well drilling platforms, presently utilize conductors for well drilling operations as well as for the subsequent production of fluids such as crude oil and gas.
In the instance of any metallic structure which is submerged in an offshore body of water, the problem of electrolytic corrosion of metallic parts will be prevalent. In such an electrolytic action, some submerged surfaces of the structure act as anodes whereas other parts function as cathodes. Over a period of time, the anode segments will tend to erode and deteriorate whereas the cathodic parts will not be similarly effected.
One way to minimize or to completely avoid electrolytic corrosion in off shore structures of this nature, is through the use of sacrificial anodes. These are normally formed of zinc, aluminum, aluminum alloys, magnesium, or a similar metal which is capable of providing the desired potential. Thus, over a period of time the sacrificial body which is chemically active with sea water, will be dissolved or eaten away and have to be replaced. The main body of the metallic structure will however be unaffected.
This form of cathodic protection is well known in the offshore industry. It has been utilized for protecting many submergible metallic bodies, including the presently discussed marine platform for drilling wells, and for producing hydrocarbon products.
Many existing marine platforms which embody a jacket section as the main support component, utilize aluminum alloy sacrificial anodes which are strategically located about the submerged part of the jacket structure. These anodes will be gradually eroded away as noted above. They will, however, serve as a means of cathodic protection for the platform.
Recent trends in deep water drilling and production platforms of the type contemplated, have resulted in more massive marine structures. This leads in turn to large numbers of drilling conductors, or more dense conductor arrays to optimize fluid production. As a result, the conductors, which are normally fabricated of steel, and thus subject to corrosion, are grouped together within the jacket at a considerable distance from anode carrying structure members.
Normally, such a jacket particularly adapted to deep water use, is comprised of an open framework of steel members which carry judicially spaced sacrificial anodes. Two phenomena thereby occur which are considered detrimental to adequate cathodic protection of the drilling conductors. These include firstly, the jacket horizontal framing levels are spaced so far apart that sacrificial anodes placed at these levels have inadequate current (throw) to protect steel members located between the levels and beyond the anodes range.
Secondly, due to the physical congestion prompted by the large number of drilling conductors within a relatively small area, interior conductors in the array are shielded from the protective effects of the sacrificial anodes positioned on external or peripheral conductors. Thus, no appreciable electrolytic ion exchange occurs between the sacrificial anodes and the shielded or internal conductors in the array. This circumstance results in relatively poor or no cathodic protection whatsoever for the latter.
The offshore drilling industry has appreciated and approached the problem of drilling conductor corrosion. Further, it has succeeded to a degree in countering the problem. The attempted solutions have been addressed principally to elimination, or at least reduction of the aforementioned detrimental effects. These approaches extend across a wide range including conservative cathodic protection systems which rely on the use of many more anodes than are necessary. This method, however, is found to be expensive even though preinstalled, and can be ineffective as well.
An alternative method of applying the desired protection is through use of expensive in-situ installation of the sacrificial anodes on or around conductors after the platform has been immersed at its offshore site. Both these methods tend toward excessive costs, are often remedial in nature and frequently unsatisfactory in performance.